Polyolefin films, which are used to package liquids, must be very tough and the edges must bond together very strongly in a bag or pouch. Examples of such films can be found in U.S. Pat. Nos. 4,503,102; 4,521,437; 5,972,443; 6,256,966; 6,406,765; 6,416,833 and 6,767,599. A number of patents held by Dow in this area include U.S. Pat. Nos. 5,364,486; 5,508,051; 5,721,025; 5,879,768; 5,942,579; and 6,117,465. All of these patents describe various polymer blends which are used to manufacture flexible packages such as those described herein. An Exxon Mobil patent in this area is U.S. Pat. No. 5,206,075. The disclosures of all of these documents are incorporated herein by reference
Flex crack resistance is an extremely important property for films used in bags and pouches that are used to package flowable materials, particularly liquids, and most particularly for lower viscosity liquids like water, milk, juices, concentrates, purees and the like. These liquids can slosh around considerably during handling, transportation and distribution of filled packages, causing flexing of the film and flex cracking of film materials.
Flex cracking is caused by the movement of the liquid within the pouch or bag, and is most likely to happen where the film is in close proximity to the upper surface of the liquid. Flex cracking can occur during shipping and handling of large bulk bags to the smallest fluid-containing pouches. Flex crack pinholes result in at least, loss of oxygen and moisture barrier, reducing the shelf life potential of the packaged product, and in more extreme cases, loss of the hermetic seal, rendering the product unsafe for consumption. Flex crack resistance is measured by Gelbo Flex Testing according to ASTM F392. Generally, a film with good flex crack resistance will develop no or very few pinholes when flexed for a large number of cycles with the Gelbo flex tester.
Thermal resistance of films is an important factor in aseptic packaging, in particular for aseptic steam sterilization filling processes for bags and pouches. Bags made from films with low thermal resistance tend to exhibit wrinkling or so called “Crocodile Skin” on the exterior of the bag after steam sterilization, resulting in poor aesthetics and bag properties. This wrinkling can often be accompanied by the inner and outer plies of a multiply bag sticking together, or even a bag or pouch made from a monolayer film sticking to itself. In a typical steam sterilization process for aseptic filling of liquid foods, the bag is first placed into a drum or bin and the fitment is then secured onto the fill head of the filling machine. Before the fitment on the bag is opened, the fitment (or spout) assembly is subjected to a steam flush ranging from 3 to 60 seconds. The fitment is then opened, and the product is pumped into the bag. At this stage, steam can enter the bag. Residual steam in the fill head keeps the temperature at about saturated steam conditions. Once the bag has been filled with product, a steam flush is employed before, or while the fitment is being closed. This post-fill steam flush can typically last from 2 to 8½ seconds. During this step, steam often enters the bag. The higher the steam temperature used in these filling operations, the greater the chance of wrinkling of the bag and hence the need for a more thermally resistant film for the bag.
Films with inadequate thermal resistance may stretch and deform unacceptably in close proximity to heated machine parts such as sealing jaws found in form, fill and seal machines. The stretched or deformed area of the film may become the weak point of the pouch or bag, at which it will fail prematurely in subsequent shipping and handling.
The sterilization cycle for aseptic form, fill, and seal pouch machines commonly uses steam at temperatures above 100° C. for sterilizing the fill tubes. Films used to produce pouches, which have polyethylene inner layers, cannot withstand these temperatures and stick to the fill tubes. To eliminate sticking of the production film, a special thermally resistant leader film must be used during the sterilization cycle. The films of this invention offer more thermal resistance and the potential to eliminate special leader films.
Other film applications also require good thermal resistance, such as hot-fill applications. In this type of operation, the product is hot as it is pumped into the bag or pouch—typically from 77 to 96° C. The heat from the product serves to sterilize the inside of the bag or pouch and fitment. The bag or pouch, once filled, slides down along an inclined chute and is flipped so that the fitment is facing down (with hot product above it). The bag or pouch then passes through a long heating tunnel for several minutes which is maintained at roughly the same temperature as the product fill temperature to keep the contents hot and to kill mold and bacteria. The bag or pouch subsequently enters and passes through a long cooling tunnel to cool down to almost room temperature. Films used in such bags or pouches require good thermal resistance so that the films and the seals maintain their integrity while in contact with the hot product.
Thermal resistance of films is assessed in a number of ways. Generally, Hot Tack Initiation Temperature and Hot Tack Strength are the primary indicators of a film's thermal resistance. Hot Tack is the strength of the molten seal immediately after sealing before it has cooled down to ambient temperature and achieved its final strength. Hot Tack Initiation Temperature is the minimum temperature at which 2N/inch of Hot Tack Strength is achieved in the Hot Tack Test as described below. The Hot Tack Initiation Temperature, sometimes referred to as Seal Initiation Temperature, is the minimum temperature required to form a molten seal of significant strength. It generally cools to form a low strength seal that can be peeled apart without stretching or distorting the film. This property allows the prediction of molten seal strength and resistance to thermal deformation (wrinkling) and/or sticking.
Prior art patent and non-patent literature contains numerous disclosures, which claim to compatibilize polyethylene and polypropylene in a functional way. In this regard, reference may be had to US Patent Publication No. 2002/0006482 which contains an extensive review of the patent and literature publications in this area. However, to date no adequate direction has been provided to the packaging art, in particular the liquid packaging art, as to how to select a suitable subset of polypropylenes that can be used in combination with polyethylene to form functional liquid packages, which meet all of the industry criteria.